Currently, nonaqueous electrolyte secondary batteries represented by lithium ion secondary batteries, particularly lithium secondary batteries, are widely mounted on portable terminals, and so on. For these nonaqueous electrolyte secondary batteries, principally LiCoO2 is used as a positive active material. However, the discharge capacity of LiCoO2 is about 120 to 130 mAh/g.
As a material of a positive active material for a lithium secondary battery, a solid solution of LiCoO2 and other compounds are known. Li[Co1-2xNixMnx]O2 (0<x≦½), a solid solution having an α-NaFeO2-type crystal structure and formed of three components: LiCoO2, LiNiO2 and LiMnO2, was published in 2001. LiNi1/2Mn1/2O2 or LiCo1/3Ni1/3Mn1/3O2 that is one example of the aforementioned solid solution has a discharge capacity of 150 to 180 mAh/g, and is also excellent in terms of charge-discharge cycle performance.
In contrast with so called a “LiMeO2-type” active material as described above, so called a “lithium-excess-type” active material is known in which the composition ratio Li/Me of lithium (Li) to the ratio of a transition metal (Me) is greater than 1, with Li/Me being, for example, 1.25 to 1.6 (see, for example, U.S. Pat. Nos. 6,677,082, 7,135,252, JP-A-10-106543 and JP-A-2010-86690). This material can be denoted as Li1+αMe1−αO2 (α>0). Here, β=(1+α)/(1−α) when the composition ratio Li/Me of lithium (Li) to the ratio of a transition metal (Me) is β, and therefore, for example, α=0.2 when Li/Me is 1.5.
U.S. Pat. Nos. 6,677,082 and 7,135,252 describe an active material for a lithium secondary battery, which has a general formula of xLiMO2.(1−x)Li2M′O3(0<x<1). The documents also describe that M is at least one selected from Mn, Co and Ni and that M′ is Mn. The documents show that the active material enriched in Li has a stabilized crystal structure, and a lithium secondary battery having a high discharge capacity is obtained by using this active material.
JP-A-10-106543 describes “a lithium battery comprising a positive active material which is formed of a composite oxide having a composition represented by LiXMeYA(1-Y)O(1+X) (where 1.3≦X≦2.5, 0.5≦Y≦0.999) where Me is at least one transition metal selected from 7A and 8A groups of the periodic table, Mt is a transition metal different from Me, and A is at least one element selected from the group consisting of Mt, Na, K, Rb, Cs, Al, Ga, In, Tl, B, Mg, Ca, Sr, Ba and Pb, and having a hexagonal crystal structure” (claim 1). The document shows that the positive active material enriched in Li has a stabilized crystal structure, and a lithium battery having a high energy density is obtained by using this positive active material. The document also shows as Example a positive active material wherein x is 1.3, Me is Mn and A is Co.
JP-A-2010-86690 describes the invention of “an active material for a lithium secondary battery comprising a solid solution of a lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, wherein the composition ratio of Li, Co, Ni and Mn contained in the solid solution satisfies Li1+1/3xCo1-x-yNiy/2Mn2x/3+y/2 (x+y≦1, 0≦y, 1−x−y=z), (x, y, z) is represented by a value present on the line of or within a heptagon ABCDEFG having point A (0.45, 0.55, 0), point B (0.63, 0.37, 0), point C (0.7, 0.25, 0.05), point D (0.67, 0.18, 0.15), point E (0.75, 0, 0.25), point F (0.55, 0, 0.45) and point G (0.45, 0.2, 0.35) as apexes, in a Li[Li1/3Mn2/3]O2(x)-LiNi1/2Mn1/2O2(y)-LiCoO2(z)-system triangular phase diagram, and the intensity ratio of the diffraction peak of the (003) line and the (104) line in X-ray diffraction measurement is I(003)/I(104)≧1.56 before charge-discharge, and I(003)/I(104)>1 at the end of discharge” (claim 1). The document shows that by using the active material enriched in Li, a lithium secondary battery, which has a high discharge capacity, and particularly has a high discharge capacity in a potential range of 4.3 V or less, is obtained.
On the other hand, it is also known that a positive active material for a lithium secondary battery, which contains a lithium transition metal composite oxide formed of Li and transition metal elements (Co, Ni, Mn and the like) contains an alkali component (see JP-A-2011-124086, JP-A-2009-140787 and International Publication No. WO 2012/039413).
JP-A-2011-124086 describes the invention of “a positive active material for a lithium secondary battery which comprises a lithium composite oxide represented by the following formula (I): Li(x)Ni(1-a-b)Co(a)Mn(b)O2 (1) (where x is 0.98≦x≦1.20, a is 0<a≦0.5, and b is 0<b≦0.5), wherein the amount of residual alkali present on the surface part of a primary particle is 4000 ppm or less, and the amount of sulfate radicals present on the surface part of the primary particle is 500 to 11000 ppm” (claim 1), and “a positive active material for a lithium secondary battery which is obtained by a first sintering step of sintering at 950° C. or lower a sintering raw material mixture containing a lithium compound, a nickel compound, a cobalt compound and a manganese compound to obtain a lithium composite oxide represented by the following formula (I): Li(x)Ni(1-a-b)Co(a)Mn(b)O2 (1) (where x is 0.98≦x≦1.20, a is 0<a≦0.5, and b is 0<b≦0.5); an aqueous sulfate solution treatment step comprising washing and contacting with an aqueous sulfate solution the lithium composite oxide obtained in the first sintering step and represented by the general formula (1), so that an aqueous sulfate solution treatment product is obtained; and a second sintering step comprising sintering the aqueous sulfate solution treatment product at 400 to 800° C. to obtain a positive active material for a lithium secondary battery” (claim 4). An object of the invention is to “provide a lithium nickel cobalt manganese-based composite oxide which has a reduced amount of residual alkali present on the surface part of a primary particle and is excellent in cycle performance” (paragraph [0011]).
JP-A-2011-124086 describes that “for the amount of residual alkali present on the surface part of a primary particle of a positive active material, 5 g of a sample and 100 g of ultrapure water were weighed and taken in a beaker, and dispersed at 25° C. for 5 minutes using a magnetic stirrer; then, the dispersion was filtered, 30 ml of the filtrate was titrated with 0.1 N-HCl by an automatic titrator (model: COMTITE-2500), and the amount of residual alkali present in the sample (value obtained by measuring the amount of lithium and calculating it into the amount of lithium carbonate) was calculated.” (paragraph [0103]).
JP-A-2009-140787 describes the invention of “a positive active material used in a nonaqueous electrolyte secondary battery, wherein the positive active material is a lithium-nickel-cobalt-manganese composite oxide, and the lithium-nickel-cobalt-manganese composite oxide contains tungsten and niobium” (claim 1) and “the positive active material according to any one of claims 1 to 3, wherein the content of the water-soluble alkali contained in the lithium-nickel-cobalt-manganese composite oxide is 0.2 wt % or less” (claim 4). An object of the invention is to “provide a positive active material which has an excellent power characteristic and generates a reduced amount of gas, and a battery using the positive active material” (paragraph [0009]).
JP-A-2009-140787 also describes that “50 ml of pure water is added to 10 g of a positive active material, and the resulting mixture is stirred for an hour, and then filtered; the filtrate is diluted to an appropriate concentration, followed by adding phenolphthalein as an indicator, and carrying out titration with a H2SO4 solution; the weight ratio of lithium hydroxide to the positive active material from the result of titration on the presumption that the alkali neutralized with the H2SO4 solution is all lithium hydroxide; and this value is defined as a content of water-soluble alkali” (paragraph [0056]).
International Publication No. WO 2012/039413 describes the invention of “an active material for a lithium secondary battery, comprising a solid solution of a sodium-containing lithium transition metal composite oxide having an α-NaFeO2-type crystal structure, wherein the chemical composition formula of the solid solution satisfies Li1+x-yNayCoaNibMncO2+d (0<y≦0.1, 0.4≦c≦0.7, x+a+b+c=1, 0.1≦x≦0.25, −0.2≦d≦0.2), the active material has an X-ray diffraction pattern attributable to a hexagonal crystal (space group P3112), and in the Miller index hkl, the half width of the diffraction peak of the (003) is 0.30° or less and the half width of the diffraction peak of the (114) line is 0.50° or less” (claim 1). The document shows that according to the invention, “an active material for a lithium secondary battery, which has a high initial efficiency and a high discharge capacity, and particularly has a high discharge capacity at a low temperature, can be provided” (paragraph [0038]). Also, it is shown as Example that the content of Na (value of y described above) is set at 0.01 to 0.1 mol (see Table 1) by a method using a coprecipitation hydroxide precursor.
The discharge capacity of so called a “lithium-excess-type” active material as described above is generally higher than that of so called a “LiMeO2-type” active material. In recent years, however, an active material with a further high discharge capacity has been required for lithium secondary batteries that are used in the field of automobiles such as electric cars, hybrid cars and plug-in hybrid cars.